1. Field of the Invention
This invention relates to a display apparatus, and more particularly to a display apparatus for displaying images based on video signals received at least from an image reproducing apparatus such as a video tape recorder (VTR).
2. Description of the Prior Art
FIG. 3 shows a liquid crystal display (LCD) panel of an active matrix type LCD apparatus, which is a typical example of a display apparatus. On one side of a first glass substrate 1A, a number of gate bus lines and source bus lines (not shown) are formed in such a manner that they cross one another at right angles. Picture element electrodes 2 which are made of a transparent conductive material, and thin film transistors (TFTS) 3 are formed at each of the crossings of the gate and source bus lines to form a matrix arrangement. The gate and source of each TFT 3 are connected to the corresponding gate bus line and corresponding source bus line, respectively, and the drain is connected to the adjacent picture element electrode 2. The top of the formative surface of the picture element electrodes 2 and TFTs 3 is coated with a transparent protective film 4 and an orientation film 5A.
A second glass substrate 1B is disposed opposite the first glass substrate 1A. On the one side of this glass substrate 1B which faces the glass substrate 1A, a black matrix 6 is formed for shielding from light the areas other than the picture element electrodes 2 of the glass substrate 1A. Red, green and blue color filters 7 are formed correspondingly to the picture element electrodes 2 on the black matrix 6. A common electrode 8 which is made of a transparent conductive material is formed over the entire surface of the color filters 7, and is covered by an orientation film 5B.
Between the orientation film 5A and the orientation film 5B, a liquid crystal layer 9 is disposed. Polarization plates 10A and 10B are arranged respectively on the one side of the first glass substrate 1A opposite to the formative surface of the picture element electrodes 2, etc., and on the one side of the second glass substrate 1B opposite to the formative surface of the color filter 7, etc.
In the LCD apparatus comprising the above-described LCD panel, a scanning voltage (the voltage signal which designates in sequence each horizontal row of the picture element electrodes 2) is sequentially applied to the gate bus lines to turn on the TFTs 3 connected to the applied gate bus line. Drive voltages corresponding respectively to the display data for the scanned horizontal row of the picture elements are supplied from the source bus lines to be applied to the horizontal row of the picture element electrodes 2 through the turned-on TFTs 3.
When the drive voltage is applied to one of the picture element electrodes 2 through the corresponding TFT 3, the drive voltage is selectively applied to the part of the liquid crystal layer 9 (this part constitutes one picture element) which is sandwiched between the driven picture element electrode 2 and the common electrode 8, causing the optical characteristics of the part to change. When this change in the optical characteristics occurs, light is transmitted through only the driven picture element (the normally black system), or light is transmitted through only the picture elements which are not driven (the normally white system), depending upon the combination of the angles of the polarization axis of the polarization plates 10A and 10B. In the normally black system, light is transmitted through driven picture elements so that color display is performed for the color tones of color filters 7 each corresponding to those picture elements, while light is not transmitted through the picture elements when they are not driven and those picture elements display black. In the case of the normally white system, it is the opposite of this.
In a display apparatus using a cathode-ray tube, the level of the beam current varies depending upon input signal levels, thereby causing the overall size of the displaying area in the vertical direction to be changed. In an extreme case, the vertical retrace line appears on the screen. In such a display apparatus, therefore, the size of the displaying area is set to be slightly greater than the screen.
In an LCD apparatus comprising the abovedescribed structure, on the other hand, the display is not affected by input signal levels (in other words, the size of the displaying area is fixed depending upon the logic of the display operation). Usually, therefore, the displaying area of an LCD apparatus is greater than that of a display apparatus using a cathode-ray tube.
Generally, a VTR has a plurality of reproducing heads, use of the heads being switched with the period of one vertical interval. The changeover of the reproducing heads requires some period of time. When video signals are reproduced in such a VTR, therefore, switching noises appear in the portion of reproduced video signals which corresponds to the vertical interval. When displaying images obtained from video signals from a VTR on an LCD apparatus, switching noises appear in the top and bottom portions of the displaying area, which is greater than that of a cathode-ray tube, causing a disturbance of the picture image.
Accordingly, in a prior LCD apparatus, masks are formed to cover the picture elements where switching noises appear. More specifically, as shown in FIG. 3, a black stripe 6A is formed at the region which corresponds to the picture elements where the switching noise appears, whereby light is prevented from transmitting through this region. FIG. 4 illustrates an LCD panel in which this kind of masking is provided. In this apparatus, picture elements 12 of the uppermost one row and lowermost five rows are masked by the black stripes 6A.
When such an LCD apparatus receives video signals from a video signal outputting apparatus of another type such as a television receiver, the black stripes 6A remain masking the uppermost and lowermost portions of the picture elements 12 so that a portion of the display area does not contribute to the display. This results in that it becomes impossible to make the most of the advantages of an LCD apparatus in achieving a larger displaying area as compared with a cathode-ray tube.